Re: Another day in the ordeal of cartesianProduct
On Saturday, 27 October 2012 at 16:19:43 UTC, H. S. Teoh wrote:
[snip]
I think there is some merit to being able to declare concepts;
for
example:
// This concept matches any type that has fields that satisfy
// what's specified inside the body.
concept InputRange(T) {
bool empty; // this matches @property bool empty() too
T front;
void popFront();
}
auto myRangeBasedFunc(InputRange r) {
r.popBack();// compile-time error: InputRange does
// not define .popBack
}
This way, *both* the user of the template and the template
writer are
kept honest (i.e., they both have to conform to the
requirements of an
input range). The compiler can thus statically check the
template for
correctness *before* it ever gets instantiated.
Not only so, the compiler will be able to generate a meaningful
error
message when the templates fail to match -- it can tell the
user the
template didn't match 'cos struct X that you tried to pass to
it isn't
an InputRange, nor a ForwardRange, ... etc..
We've had a short discussion with Jacob Carlborg about almost
exactly this syntax
(http://forum.dlang.org/post/jukabm$1btd$1...@digitalmars.com) in
the context of better compiler messages.
I will quote my concerns:
=
void foo (InputRange range);
1. How would you signify `foo` is generic ?
For complier it's probably possible - by type of `InputRange`,
but that will be one more special case
What about user ?
2. How would you put 2 constraints on `range` ?
3. Also you forgot to address:
template isInputRange(R)
{
enum bool isInputRange = is(typeof(
(inout int _dummy=0)
{
R r = void; /// how would you express this
/// b.t.w. ?
range.d comment on this line is can define a range object.
= end quote
However I do agree it would be nice if compiler verifies template
body against constraints. IMNA compiler-writer, but I wonder if
it's possible with current syntax.
Re: Another day in the ordeal of cartesianProduct
On 27/10/12 00:45, H. S. Teoh wrote: http://d.puremagic.com/issues/show_bug.cgi?id=8900 :-( (The code there is called cartesianProd but it's the reduced code, so it doesn't really compute the cartesian product. But that's where it's from.) So far, the outstanding blockers for cartesianProduct are: 1) Compiler bug which causes unittest failure: std/range.d(4629): Error: variable lower used before set std/range.d(4630): Error: variable upper used before set (Jonathan had a pull request with a Phobos workaround for this, which I _think_ is already merged, but the autotester is still failing at this point. :-/) 2) Issue 8542 (crosstalk between template instantiations) 3) And now, issue 8900 (zip fails to compile with repeat(char[])) So there's still no joy for cartesianProduct. :-( I'm getting a bit frustrated with the Phobos bugs related to ranges and std.algorithm. I think we need to increase the number of unittests. And by that I mean, GREATLY increase the number of unittests. Most of the current tests are merely sanity tests for the most common usage patterns, most basic types, or tests added as a result of fixed bugs. This is inadequate. We need to actively unittest corner cases, rare combinations, unusual usages, etc.. Torture test various combinations of range constructs. Algorithms. Nested range constructs. Nested algorithms. Deliberately cook up nasty tests that try their best to break the code by using unusual parameters, unusual range-like objects, strange data, etc.. Go beyond the simple cases to test non-trivial things. We need unittests that pass unusual structs and objects into the range constructs and algorithms, and make sure they actually work as we have been _assuming_ they should. I have a feeling there are a LOT of bugs lurking in there behind overlooked corner cases, off by 1 errors, and other such careless slips, as well as code that only works for basic types like arrays, which starts breaking when you hand it something non-trivial. All these issues must be weeded out and prevented from slipping back in. Here's a start: - Create a set of structs/classes (inside a version(unittest) block) that are input, forward, bidirectional, output, etc., ranges, that are NOT merely arrays. - There should be some easy way, perhaps using std.random, of creating non-trivial instances of these things. These should be put in a separate place, perhaps outside the std/ subdirectory, where they can be imported into unittest blocks by std.range, std.algorithm, whatever else that needs extensive testing. - Use these ranges as input for testing range constructs and algorithms. - For best results, use a compile-time loop to loop over a given combination of these range types, and run them through the same set of tests. This will improve the currently spotty test coverage. Perhaps provide some templated functions that, given a set of range types (from the above structs/classes) and a set of functions, run through all combinations of them to make sure they all work. (We run unittests separately anyway, we aren't afraid of long-running tests.) T I think that unit tests aren't very effective without code coverage. One fairly non-disruptive thing we could do: implement code coverage for templates. Currently, templates get no code coverage numbers. We could do a code-coverage equivalent for templates: which lines actually got instantiated? I bet this would show _huge_ gaps in the existing test suite.
Re: Another day in the ordeal of cartesianProduct
On 10/28/12 8:28 AM, Peter Alexander wrote:
On Saturday, 27 October 2012 at 13:06:09 UTC, Andrei Alexandrescu wrote:
On 10/27/12 8:23 AM, Peter Alexander wrote:
Retrofitting some sort of structure to templates will be a Herculean
task, but I think it has to happen. It is clear to me that the
development process we use now (write the template, try a few
instantiations, pray) is unsustainable beyond simple templates.
It's not clear to me at all. The mechanism works very well and is more
expressive than alternatives used by other languages.
I'm not sure I can agree it works well.
For example, here's what happened with bug 8900 mentioned in the OP:
std.range.zip creates a Zip object, which has a Tuple member. Tuple has
a toString function, which calls formatElement, which calls formatValue,
which calls formatRange, which (when there's a range of characters) has
a code path for right-aligning the range. To right-align the range it
needs to call walkLength.
The problem arises when you zip an infinite range of characters e.g.
repeat('a').
This proves nothing at all. So this has to do with invoking walkLength
against an infinite range. At the time I wrote walkLength, infinite
ranges were an experimental notion that I was ready to remove if there
wasn't enough practical support for it. So I didn't even think of the
connection, which means the restriction wouldn't have likely made it
into the definition of walkLength regardless of the formalism used.
Before pull request 880, walkLength accepted infinite ranges and just
returned size_t.max. Pull request 880 added a constraint to walkLength
to stop it accepting infinite ranges. Suddenly, you cannot zip a range
of infinite chars because of a seemingly unrelated change.
The connection is obvious and is independent qualitatively of other
cases of if you change A and B uses it, B may change in behavior too.
It's a pattern old as dust in programming.
Anyway, I'm not sure whether this is clear as day: expressing
constraints as Booleans or C++ concepts style or Gangnam style doesn't
influence this case in the least.
This would have been caught if there was a unit test, but there wasn't,
and as Dijkstra says, testing can be a very effective way to show the
presence of bugs, but it is hopelessly inadequate for showing their
absence. There are probably other places that are broken, and many
changes in the future will just introduce more bugs without tests.
Maybe we have different standards for working well, but to me at
least, this isn't what working well looks like.
Working well in this case would look like this:
- The person that put together pull request 880 would add the template
constraint to walkLength.
- On the next compile he would get this error: formatRange potentially
calls walkLength with an infinite range. (or something along those lines).
- The person fixes formatRange, and all is well.
No need for unit tests, it's all caught as soon as possible without need
for instantiation.
But this works today and has nothing to do with retrofitting structure
to templates. Nothing. Nothing.
Andrei
Re: Another day in the ordeal of cartesianProduct
On Monday, 29 October 2012 at 14:47:37 UTC, Don Clugston wrote:
One fairly non-disruptive thing we could do: implement code
coverage for templates. Currently, templates get no code
coverage numbers.
We could do a code-coverage equivalent for templates: which
lines actually got instantiated?
I bet this would show _huge_ gaps in the existing test suite.
That's a good step forward, but I don't think it solves (what
appears to me to be) the most common issue: incorrect/missing
template constraints.
auto average(R)(R r) if (isForwardRange!R)
{
return reduce!(a + b)(r) / walkLength(r);
}
(This code can have full coverage for some ranges, but also needs
to also check !isInfinite!R, and probably other things).
These are difficult because every time a constraint changes, you
need to go round and update the constraints of all calling
functions. It's like when you change the types of a function
argument, or return type; except that with template constraints,
nothing is checked until instantiation.
Re: Another day in the ordeal of cartesianProduct
On Monday, 29 October 2012 at 15:48:11 UTC, Andrei Alexandrescu
wrote:
On 10/28/12 8:28 AM, Peter Alexander wrote:
For example, here's what happened with bug 8900 mentioned in
the OP:
std.range.zip creates a Zip object, which has a Tuple member.
Tuple has
a toString function, which calls formatElement, which calls
formatValue,
which calls formatRange, which (when there's a range of
characters) has
a code path for right-aligning the range. To right-align the
range it
needs to call walkLength.
The problem arises when you zip an infinite range of
characters e.g.
repeat('a').
This proves nothing at all. So this has to do with invoking
walkLength against an infinite range. At the time I wrote
walkLength, infinite ranges were an experimental notion that I
was ready to remove if there wasn't enough practical support
for it. So I didn't even think of the connection, which means
the restriction wouldn't have likely made it into the
definition of walkLength regardless of the formalism used.
You're misunderstanding. walkLength used to allow infinite
ranges. Recently, a commit added a constraint to walkLength to
disallow infinite ranges. After this commit, all the unit tests
still passed, but at least one bug was introduced (bug 8900).
That's the problem: a change occurred that introduced a bug, but
the type system failed to catch it before the change was
committed. Something like typeclasses would have caught the bug
before commit and without unit tests.
The connection is obvious and is independent qualitatively of
other cases of if you change A and B uses it, B may change in
behavior too. It's a pattern old as dust in programming.
Anyway, I'm not sure whether this is clear as day: expressing
constraints as Booleans or C++ concepts style or Gangnam
style doesn't influence this case in the least.
If I change A and B uses it, I expect B to give an error or at
least a warning at compile time where possible. This doesn't
happen. With template constraints, you don't get an error until
you try to instantiate the template. This is too late in my
opinion.
I would like this to give an error:
void foo(R)(R r) if (isForwardRange!R) { r.popBack(); }
It doesn't, not until you try to use it at least, and even then
it only gives you an error if you try it with a non-bidirectional
forward range. If this did give an error, bug 8900 (any many
others) would never have happened.
The problem with constraints vs. something like typeclasses or
C++ concepts is that constraint predicates are not possible to
enforce pre-instantiation. They have too much freedom of
expression.
Working well in this case would look like this:
- The person that put together pull request 880 would add the
template
constraint to walkLength.
- On the next compile he would get this error: formatRange
potentially
calls walkLength with an infinite range. (or something along
those lines).
- The person fixes formatRange, and all is well.
No need for unit tests, it's all caught as soon as possible
without need
for instantiation.
But this works today and has nothing to do with retrofitting
structure to templates. Nothing. Nothing.
It doesn't work today.
This isn't a fabricated example. This happened. walkLength
changed its constraint, everything still compiled, and all the
unit tests passed. There was no error, no hint that things were
broken, nothing. Problems only started to arise when the poor OP
tried to implement cartesianProduct.
This should never have happened. Typeclasses or C++ concepts
wouldn't have allowed it to happen. This is the kind of structure
that templates need.
Re: Another day in the ordeal of cartesianProduct
On 10/29/12 12:21 PM, Peter Alexander wrote:
On Monday, 29 October 2012 at 15:48:11 UTC, Andrei Alexandrescu wrote:
On 10/28/12 8:28 AM, Peter Alexander wrote:
For example, here's what happened with bug 8900 mentioned in the OP:
std.range.zip creates a Zip object, which has a Tuple member. Tuple has
a toString function, which calls formatElement, which calls formatValue,
which calls formatRange, which (when there's a range of characters) has
a code path for right-aligning the range. To right-align the range it
needs to call walkLength.
The problem arises when you zip an infinite range of characters e.g.
repeat('a').
This proves nothing at all. So this has to do with invoking walkLength
against an infinite range. At the time I wrote walkLength, infinite
ranges were an experimental notion that I was ready to remove if there
wasn't enough practical support for it. So I didn't even think of the
connection, which means the restriction wouldn't have likely made it
into the definition of walkLength regardless of the formalism used.
You're misunderstanding. walkLength used to allow infinite ranges.
Recently, a commit added a constraint to walkLength to disallow infinite
ranges. After this commit, all the unit tests still passed, but at least
one bug was introduced (bug 8900).
I thought I understood the matter rather well.
That's the problem: a change occurred that introduced a bug, but the
type system failed to catch it before the change was committed.
Something like typeclasses would have caught the bug before commit and
without unit tests.
Yes, but what gets ignored here is that typeclasses have a large
cognitive cost to everyone involved. I think typeclasses generally don't
pull their weight. Besides I think template constraints are more
powerful because they operate on arbitrary Boolean expressions instead
of types.
The connection is obvious and is independent qualitatively of other
cases of if you change A and B uses it, B may change in behavior
too. It's a pattern old as dust in programming.
Anyway, I'm not sure whether this is clear as day: expressing
constraints as Booleans or C++ concepts style or Gangnam style
doesn't influence this case in the least.
If I change A and B uses it, I expect B to give an error or at least a
warning at compile time where possible. This doesn't happen. With
template constraints, you don't get an error until you try to
instantiate the template.
That's also at compile time, just a tad later.
This is too late in my opinion.
I think there's a marked difference between compile-time and run-time.
Instantiation time does not make a big enough difference to bring a big
gun in the mix.
I would like this to give an error:
void foo(R)(R r) if (isForwardRange!R) { r.popBack(); }
It doesn't, not until you try to use it at least, and even then it only
gives you an error if you try it with a non-bidirectional forward range.
So them a unittest with a minimal mock forward range should be created.
I understand your concern, but please understand that typeclasses are
too big a weight for what they do.
If this did give an error, bug 8900 (any many others) would never have
happened.
How many others? (Honest question.)
The problem with constraints vs. something like typeclasses or C++
concepts is that constraint predicates are not possible to enforce
pre-instantiation. They have too much freedom of expression.
Freedom of expression is also a strength. (The problem with C++ concepts
is that they almost sunk C++.)
Working well in this case would look like this:
- The person that put together pull request 880 would add the template
constraint to walkLength.
- On the next compile he would get this error: formatRange potentially
calls walkLength with an infinite range. (or something along those
lines).
- The person fixes formatRange, and all is well.
No need for unit tests, it's all caught as soon as possible without need
for instantiation.
But this works today and has nothing to do with retrofitting
structure to templates. Nothing. Nothing.
It doesn't work today.
This isn't a fabricated example.
It's just blown out of proportion.
This happened. walkLength changed its
constraint, everything still compiled, and all the unit tests passed.
There was no error, no hint that things were broken, nothing. Problems
only started to arise when the poor OP tried to implement cartesianProduct.
This should never have happened. Typeclasses or C++ concepts wouldn't
have allowed it to happen. This is the kind of structure that templates
need.
We will not add C++ concepts or typeclasses to D.
Andrei
Re: Another day in the ordeal of cartesianProduct
Andrei Alexandrescu: Yes, but what gets ignored here is that typeclasses have a large cognitive cost to everyone involved. Such costs are an interesting discussion topic :-) To put people up to speed a bit: http://en.wikipedia.org/wiki/Type_class A bit of explanations regarding Rust ones: https://air.mozilla.org/rust-typeclasses/ Deeper info from one of the original designers: http://homepages.inf.ed.ac.uk/wadler/topics/type-classes.html I think typeclasses generally don't pull their weight. Rust and Haskell designers think otherwise, it seems. We will not add C++ concepts or typeclasses to D. I agree that maybe now it's too much late to add them to D2. But we are free to discuss about the topic. I didn't know much about typeclasses years ago when people were designing D2 :-( I have studied them only recently while learning Haskell. Bye, bearophile
Re: Another day in the ordeal of cartesianProduct
On 10/29/12 2:16 PM, bearophile wrote: Andrei Alexandrescu: Yes, but what gets ignored here is that typeclasses have a large cognitive cost to everyone involved. Such costs are an interesting discussion topic :-) To put people up to speed a bit: http://en.wikipedia.org/wiki/Type_class A bit of explanations regarding Rust ones: https://air.mozilla.org/rust-typeclasses/ Deeper info from one of the original designers: http://homepages.inf.ed.ac.uk/wadler/topics/type-classes.html For those who wouldn't know how to search the Net, these indeed are quite appropriate. I think typeclasses generally don't pull their weight. Rust and Haskell designers think otherwise, it seems. It's a matter of what priorities the language has and what other features are available overlapping with the projected usefulness. Andrei
Re: Another day in the ordeal of cartesianProduct
Andrei Alexandrescu: For those who wouldn't know how to search the Net, these indeed are quite appropriate. People are often a bit lazy in clicking on links present in newsgroup messages, and they are even more lazy in searching things. So I've seen numerous times that if you give them links they will be more willing to read. This is especially true about topics they know nothing about, because they have a higher cognitive impedance. Google gives many results about this topic, but not all of them are good. That link from mozilla is a little video that doesn't tell a lot. The papers from Wadler were the ones Haskell typeclases come from, they are harder, but they are more fundamental. But starting from those papers is hard, so better to start from some examples and simpler explanations. In the Haskell wiki there are more practical texts on this topic, and comparisons: http://www.haskell.org/tutorial/classes.html http://www.haskell.org/haskellwiki/OOP_vs_type_classes Plus an online free chapter of what maybe is the best book about Haskell: http://book.realworldhaskell.org/read/using-typeclasses.html It's a matter of what priorities the language has and what other features are available overlapping with the projected usefulness. I agree. On the other hand the main point of this thread is that someone perceives as not good enough or not sufficient the current features of D. Even if their perception is wrong (and you are an expert on this field, so we trust your words), I think the topic is worth discussing. Bye, bearophile
Re: Another day in the ordeal of cartesianProduct
On Monday, 29 October 2012 at 17:56:26 UTC, Andrei Alexandrescu wrote: We will not add C++ concepts or typeclasses to D. Ok. There is no point continuing this discussion then.
Re: Another day in the ordeal of cartesianProduct
On Saturday, 27 October 2012 at 13:06:09 UTC, Andrei Alexandrescu
wrote:
On 10/27/12 8:23 AM, Peter Alexander wrote:
Retrofitting some sort of structure to templates will be a
Herculean
task, but I think it has to happen. It is clear to me that the
development process we use now (write the template, try a few
instantiations, pray) is unsustainable beyond simple templates.
It's not clear to me at all. The mechanism works very well and
is more expressive than alternatives used by other languages.
I'm not sure I can agree it works well.
For example, here's what happened with bug 8900 mentioned in the
OP:
std.range.zip creates a Zip object, which has a Tuple member.
Tuple has a toString function, which calls formatElement, which
calls formatValue, which calls formatRange, which (when there's a
range of characters) has a code path for right-aligning the
range. To right-align the range it needs to call walkLength.
The problem arises when you zip an infinite range of characters
e.g. repeat('a').
Before pull request 880, walkLength accepted infinite ranges and
just returned size_t.max. Pull request 880 added a constraint to
walkLength to stop it accepting infinite ranges. Suddenly, you
cannot zip a range of infinite chars because of a seemingly
unrelated change.
This would have been caught if there was a unit test, but there
wasn't, and as Dijkstra says, testing can be a very effective
way to show the presence of bugs, but it is hopelessly inadequate
for showing their absence. There are probably other places that
are broken, and many changes in the future will just introduce
more bugs without tests.
Maybe we have different standards for working well, but to me
at least, this isn't what working well looks like.
Working well in this case would look like this:
- The person that put together pull request 880 would add the
template constraint to walkLength.
- On the next compile he would get this error: formatRange
potentially calls walkLength with an infinite range. (or
something along those lines).
- The person fixes formatRange, and all is well.
No need for unit tests, it's all caught as soon as possible
without need for instantiation.
Re: Another day in the ordeal of cartesianProduct
On Sunday, 28 October 2012 at 12:28:23 UTC, Peter Alexander wrote: Before pull request 880, walkLength accepted infinite ranges and just returned size_t.max. Actually, before 880, you had an infinite loop, as size_t.max was a magic number that meant walk to end... I've found recently trying to do coverage for std.container and others, that an efficient method of unittesting is to just try to instanciate the template in as many forms as possible. While you aren't actually verifying any run-time behavior, you ARE finding a lot of compile errors. I haven't commited any of them, but I have been able to detect and fix quite some errors. They are [keyboar died]
Re: Another day in the ordeal of cartesianProduct
On Sunday, 28 October 2012 at 14:11:41 UTC, monarch_dodra wrote: ...[keyboar died] Yeah, sorry. I was saying that by just doing unit tests that do nothing more than declare the templates, you can get some very good compile coverage. It is not optimal, but it gives a good bang for the buck, in terms of invested effort (very little, just typing the types, but not testing anything).
Re: Another day in the ordeal of cartesianProduct
On Friday, 26 October 2012 at 22:43:44 UTC, H. S. Teoh wrote: This is inadequate. The best we can get are users like you that take the time to report ;) Thanks.
Re: Another day in the ordeal of cartesianProduct
On Friday, 26 October 2012 at 22:43:44 UTC, H. S. Teoh wrote:
I'm getting a bit frustrated with the Phobos bugs related to
ranges and
std.algorithm. I think we need to increase the number of
unittests. And
by that I mean, GREATLY increase the number of unittests. Most
of the
current tests are merely sanity tests for the most common usage
patterns, most basic types, or tests added as a result of fixed
bugs.
This is inadequate.
Unit tests are not the solution to this. There are too many range
combinations and types to effectively unit test everything. What
we need is proper semantic support for ranges (and other
conceptual classes of types). For example, this should not
compile:
void foo(Range)(Range r) if (isForwardRange!Range)
{
r.popBack();
}
But it will, because of the way templates work in D -- nothing is
checked until you try to use it.
Imagine I could compile non-template code like this:
void foo(int x)
{
writeln(x.name);
}
Imagine you had to try to call it before the static type error
was caught. This is not a way to write robust code, especially
with the combinatorial explosion of range types that exist.
Template constraints are nice, but not enough. We really need
something *like* the proposed C++ concepts, or just something
analogous to interfaces. I should be able to write something like
this:
void foo(ForwardRange Range)(Range r)
{
r.popBack();
}
And *immediately* get a type error without having to instantiate
it with a variety of different range types. This is the only way
I can see these kinds of problems going away. Unit testing does
not scale with exponential use cases.
Re: Another day in the ordeal of cartesianProduct
Peter Alexander:
void foo(ForwardRange Range)(Range r)
{
r.popBack();
}
And *immediately* get a type error without having to
instantiate it with a variety of different range types.
It's not an easy problem. To solve it the Rust language uses
typeclasses adapted from Haskell. But unlike the usual Haskell
compilers Rust doesn't use global type inferencing and it keeps
the C++-style monomorphization, so at run-time its generic
programming is as efficient as C++ generic programming.
I also hope Rust designers will take a look at this (Efficient
Dynamic Dispatch without Virtual Function Tables. The SmallEiffel
Compiler), an alternative to virtual tables. Maybe the D
front-end could do the same on request with a compilation switch
(with no changes in D language):
http://smarteiffel.loria.fr/papers/oopsla97.pdf
Bye,
bearophile
Re: Another day in the ordeal of cartesianProduct
On Saturday, 27 October 2012 at 11:41:07 UTC, bearophile wrote:
Peter Alexander:
void foo(ForwardRange Range)(Range r)
{
r.popBack();
}
And *immediately* get a type error without having to
instantiate it with a variety of different range types.
It's not an easy problem. To solve it the Rust language uses
typeclasses adapted from Haskell. But unlike the usual Haskell
compilers Rust doesn't use global type inferencing and it keeps
the C++-style monomorphization, so at run-time its generic
programming is as efficient as C++ generic programming.
Yeah, it's certainly not going to be easy. It's unfortunate that
D adopted the whole C++ style glorified macros approach to
templates -- it makes it very difficult to reason about (or
automate reasoning about) the semantics of your code.
Retrofitting some sort of structure to templates will be a
Herculean task, but I think it has to happen. It is clear to me
that the development process we use now (write the template, try
a few instantiations, pray) is unsustainable beyond simple
templates.
Re: Another day in the ordeal of cartesianProduct
On 10/27/12 6:20 AM, Peter Alexander wrote:
On Friday, 26 October 2012 at 22:43:44 UTC, H. S. Teoh wrote:
I'm getting a bit frustrated with the Phobos bugs related to ranges and
std.algorithm. I think we need to increase the number of unittests. And
by that I mean, GREATLY increase the number of unittests. Most of the
current tests are merely sanity tests for the most common usage
patterns, most basic types, or tests added as a result of fixed bugs.
This is inadequate.
Unit tests are not the solution to this. There are too many range
combinations and types to effectively unit test everything.
Agreed.
What we need
is proper semantic support for ranges (and other conceptual classes of
types). For example, this should not compile:
void foo(Range)(Range r) if (isForwardRange!Range)
{
r.popBack();
}
No, this has nothing to do with the problem at hand.
Andrei
Re: Another day in the ordeal of cartesianProduct
On 10/26/12 6:45 PM, H. S. Teoh wrote: http://d.puremagic.com/issues/show_bug.cgi?id=8900 :-( (The code there is called cartesianProd but it's the reduced code, so it doesn't really compute the cartesian product. But that's where it's from.) So far, the outstanding blockers for cartesianProduct are: 1) Compiler bug which causes unittest failure: std/range.d(4629): Error: variable lower used before set std/range.d(4630): Error: variable upper used before set (Jonathan had a pull request with a Phobos workaround for this, which I _think_ is already merged, but the autotester is still failing at this point. :-/) 2) Issue 8542 (crosstalk between template instantiations) 3) And now, issue 8900 (zip fails to compile with repeat(char[])) So there's still no joy for cartesianProduct. :-( Often when there are many issues caused by a specific artifact, there are only a couple of compiler bugs manifesting themselves in various unpredictable ways. I'm getting a bit frustrated with the Phobos bugs related to ranges and std.algorithm. I think we need to increase the number of unittests. And by that I mean, GREATLY increase the number of unittests. I don't think that's the best way. Walter and I have had a lasting disagreement about this as he firmly believes unittests are the way to make sure the compiler works well. In my opinion the psychological effect has been negative because this outlook has caused incompletely-defined features to get implemented (in a latent expectation that unittests will cause any issues to be detected). Andrei
Re: Another day in the ordeal of cartesianProduct
On 10/27/12 8:23 AM, Peter Alexander wrote: Yeah, it's certainly not going to be easy. It's unfortunate that D adopted the whole C++ style glorified macros approach to templates -- it makes it very difficult to reason about (or automate reasoning about) the semantics of your code. I think D has reached a sweet spot with restricted templates. Retrofitting some sort of structure to templates will be a Herculean task, but I think it has to happen. It is clear to me that the development process we use now (write the template, try a few instantiations, pray) is unsustainable beyond simple templates. It's not clear to me at all. The mechanism works very well and is more expressive than alternatives used by other languages. Andrei
Re: Another day in the ordeal of cartesianProduct
On Sat, Oct 27, 2012 at 09:06:11AM -0400, Andrei Alexandrescu wrote:
On 10/27/12 8:23 AM, Peter Alexander wrote:
Yeah, it's certainly not going to be easy. It's unfortunate that D
adopted the whole C++ style glorified macros approach to templates
-- it makes it very difficult to reason about (or automate reasoning
about) the semantics of your code.
I think D has reached a sweet spot with restricted templates.
Retrofitting some sort of structure to templates will be a Herculean
task, but I think it has to happen. It is clear to me that the
development process we use now (write the template, try a few
instantiations, pray) is unsustainable beyond simple templates.
It's not clear to me at all. The mechanism works very well and is more
expressive than alternatives used by other languages.
[...]
I think the complaint is that the template constraints enforce ducktype
requirements on the *caller*, but not on the template code itself.
That's what Peter's example was about: the signature constraint declares
that the template takes an input range, yet the template body makes use
of non-input range properties of the argument.
I think there is some merit to being able to declare concepts; for
example:
// This concept matches any type that has fields that satisfy
// what's specified inside the body.
concept InputRange(T) {
bool empty; // this matches @property bool empty() too
T front;
void popFront();
}
auto myRangeBasedFunc(InputRange r) {
r.popBack();// compile-time error: InputRange does
// not define .popBack
}
This way, *both* the user of the template and the template writer are
kept honest (i.e., they both have to conform to the requirements of an
input range). The compiler can thus statically check the template for
correctness *before* it ever gets instantiated.
Not only so, the compiler will be able to generate a meaningful error
message when the templates fail to match -- it can tell the user the
template didn't match 'cos struct X that you tried to pass to it isn't
an InputRange, nor a ForwardRange, ... etc..
Currently, if the template writer screws up, the compiler doesn't know
any better, and only when the user actually tries to pass something that
the template body can't handle, does the error manifest itself. Which
seems a bit late -- it should be the template writer who first notices
the problem (like if the compiler can statically verify the template
body and complain loudly before the code ever gets to the user). Plus,
the error message is usually a long cryptic sequence of instantiation
failures containing all sorts of internal Phobos types that the user has
no idea about. The compiler can hardly do much better, because it
doesn't have enough information to make a meaningful message.
T
--
Старый друг лучше новых двух.
Re: Another day in the ordeal of cartesianProduct
Am Sat, 27 Oct 2012 12:20:51 +0200
schrieb Peter Alexander peter.alexander...@gmail.com:
On Friday, 26 October 2012 at 22:43:44 UTC, H. S. Teoh wrote:
I'm getting a bit frustrated with the Phobos bugs related to
ranges and
std.algorithm. I think we need to increase the number of
unittests. And
by that I mean, GREATLY increase the number of unittests. Most
of the
current tests are merely sanity tests for the most common usage
patterns, most basic types, or tests added as a result of fixed
bugs.
This is inadequate.
Unit tests are not the solution to this. There are too many range
combinations and types to effectively unit test everything.
Yes and no. We can't test all possible combinations, but we should try
to at least test all code paths:
--
void test(R)(R range)
{
static if(isInputRange!R)
r.popFront();
else if(isForwardRange!R)
r.saev();
}
unittest
{
//Should test at least with InputRange ForwardRange
}
---
We sometimes even have templates that are not used in phobos at all
(and don't have unittests). Those are really evil as they can break
silently if some typo is introduced.
Another day in the ordeal of cartesianProduct
http://d.puremagic.com/issues/show_bug.cgi?id=8900 :-( (The code there is called cartesianProd but it's the reduced code, so it doesn't really compute the cartesian product. But that's where it's from.) So far, the outstanding blockers for cartesianProduct are: 1) Compiler bug which causes unittest failure: std/range.d(4629): Error: variable lower used before set std/range.d(4630): Error: variable upper used before set (Jonathan had a pull request with a Phobos workaround for this, which I _think_ is already merged, but the autotester is still failing at this point. :-/) 2) Issue 8542 (crosstalk between template instantiations) 3) And now, issue 8900 (zip fails to compile with repeat(char[])) So there's still no joy for cartesianProduct. :-( I'm getting a bit frustrated with the Phobos bugs related to ranges and std.algorithm. I think we need to increase the number of unittests. And by that I mean, GREATLY increase the number of unittests. Most of the current tests are merely sanity tests for the most common usage patterns, most basic types, or tests added as a result of fixed bugs. This is inadequate. We need to actively unittest corner cases, rare combinations, unusual usages, etc.. Torture test various combinations of range constructs. Algorithms. Nested range constructs. Nested algorithms. Deliberately cook up nasty tests that try their best to break the code by using unusual parameters, unusual range-like objects, strange data, etc.. Go beyond the simple cases to test non-trivial things. We need unittests that pass unusual structs and objects into the range constructs and algorithms, and make sure they actually work as we have been _assuming_ they should. I have a feeling there are a LOT of bugs lurking in there behind overlooked corner cases, off by 1 errors, and other such careless slips, as well as code that only works for basic types like arrays, which starts breaking when you hand it something non-trivial. All these issues must be weeded out and prevented from slipping back in. Here's a start: - Create a set of structs/classes (inside a version(unittest) block) that are input, forward, bidirectional, output, etc., ranges, that are NOT merely arrays. - There should be some easy way, perhaps using std.random, of creating non-trivial instances of these things. These should be put in a separate place, perhaps outside the std/ subdirectory, where they can be imported into unittest blocks by std.range, std.algorithm, whatever else that needs extensive testing. - Use these ranges as input for testing range constructs and algorithms. - For best results, use a compile-time loop to loop over a given combination of these range types, and run them through the same set of tests. This will improve the currently spotty test coverage. Perhaps provide some templated functions that, given a set of range types (from the above structs/classes) and a set of functions, run through all combinations of them to make sure they all work. (We run unittests separately anyway, we aren't afraid of long-running tests.) T -- How are you doing? Doing what?
Re: Another day in the ordeal of cartesianProduct
On Sat, Oct 27, 2012 at 01:58:14AM +0200, Andrej Mitrovic wrote: On 10/27/12, H. S. Teoh hst...@quickfur.ath.cx wrote: Here's a start: - Move the tests outside the implementation modules I do not want to see another std.datetime in Phobos. :) Good idea. That way we can do cross-module testing as well. T -- EMACS = Extremely Massive And Cumbersome System
Re: Another day in the ordeal of cartesianProduct
On Friday, October 26, 2012 15:45:48 H. S. Teoh wrote: (Jonathan had a pull request with a Phobos workaround for this, which I _think_ is already merged, but the autotester is still failing at this point. :-/) It hasn't been merged yet: https://github.com/D-Programming-Language/phobos/pull/854 It's likely going to need a few more tweaks (as discussed in the pull request) before it gets merged in. But it's critical enough that there's pretty much no way that it won't be merged in before the next release, and it'll probably be merged in sooner rather than later, but that'll depend on the feedback for the request and whatever issues come up. - Jonathan M Davis
